The present invention relates to a cathode ray tube (CRT), and particularly to a cathode ray tube having an electron gun capable of improving focus characteristics, correcting deflection defocusing and thereby providing good resolution over the entire phosphor screen and over the entire electron beam current region.
A cathode ray tube such as a picture tube or a display tube includes at least an electron gun having a plurality of electrodes and a phosphor screen (a screen having a phosphor film, which is also referred to as "a phosphor film" or simply to "a screen" hereinafter), and it also includes a deflection device for scanning an electron beam emitted from the electron gun over the phosphor screen.
For such a cathode ray tube, there have been known the following techniques for obtaining a good reproduced image over the entire phosphor screen from the center to the peripheral portions.
Japanese Patent Publication No. Hei 4-52586 discloses an electron gun emitting three in-line electron beams in which a pair of parallel flat electrodes are disposed on the bottom face of a shield cup in such a manner as to be positioned above and below paths of the three electron beams in parallel to the in-line direction and to extend toward a main lens.
U.S. Pat. No. 4,086,513 and its corresponding Japanese Patent Publication No. Sho 60-7345 discloses an electron gun emitting three in-line electron beams in which a pair of parallel flat electrodes are disposed above and below paths of the three electron beams in parallel to the in-line direction in such a manner as to extend from one of facing ends of one of a pair of main-lens-forming electrodes toward a phosphor screen, thereby shaping the electron beams before the electron beams enter a deflection magnetic field.
Japanese Patent Laid-open No. Sho 51-61766 discloses an electron gun in which an electrostatic quadrupole lens is formed between two electrodes and the strength of the electrostatic quadrupole lens is made to vary dynamically with the deflection of an electron beam, thereby achieving uniformity of an image over the entire screen.
Japanese Patent Publication No. Sho 53-18866 discloses an electron gun in which an astigmatic lens is provided in a region between a second grid electrode and a third grid electrode forming a prefocus lens.
U.S. Pat. No. 3,952,224 and its corresponding Japanese Patent Laid-open No. Sho 51-64368 discloses an electron gun emitting three in-line electron beams in which an electron beam aperture of each of first and second grid electrodes is formed in an elliptic shape, and the degree of ellipticity of the aperture is made to differ for each beam path or the degree of ellipticity of the electron beam aperture of the center electron gun is made smaller than that of the side electron gun.
Japanese Patent Laid-open No. Sho 60-81736 discloses an electron gun emitting three in-line electron beams in which a slit recess provided in a third grid electrode on the cathode side forms a non-axially-symmetrical lens, and an electron beam is made to impinge on the phosphor screen through at least one non-axially-symmetrical lens in which the axial depth of the slit recess is larger for the center beam than for the side beam.
Japanese Patent Laid-open No. Sho 54-139372 discloses a color cathode ray tube having an electron gun emitting three in-line electron beams in which a soft magnetic material is disposed in fringe portions of the deflection magnetic field to form a pincushion-shaped magnetic field for deflecting the electron beams in the direction perpendicular to the in-line direction of each electron beam, thereby suppressing a halo caused by the deflection magnetic field in the direction perpendicular to the in-line direction.
The desired focus characteristics of a cathode ray tube include good resolution over the entire screen and over the entire electron beam current region; no appearance of moire in a small-current region; and uniformity in resolution over the entire screen and over the entire electron beam current region. The design of an electron gun for simultaneously satisfying these focus characteristics requires a high technique.
The present inventors found that a combination of an astigmatic lens and a large-diameter main lens is essential to obtain these focus characteristics of the cathode ray tube.
In the above-described prior art, however, a dynamic focus voltage needs to be applied to a focus electrode of an electron gun for obtaining good resolution over the entire screen using electrodes forming an astigmatic lens, that is, a non-axially-symmetrical lens in the electron gun. No consideration has been given to correction of deflection defocusing by forming a uniform magnetic field by disposing magnetic pieces in a magnetic deflection field.
FIG. 24 is a partial sectional view of one example of an electron gun used for a cathode ray tube.
The electron gun of this type comprises a plurality of electrodes including a cathode K, a first grid electrode (G1) 1, a second grid electrode (G2) 2, a third grid electrode (G3) 3, a fourth grid electrode (G4) 4, a fifth grid electrode (G5) 5, a sixth grid electrode (G6) 6, and a shield cup 30 integrally attached to the sixth electrode (G6) 6. The fifth grid electrode (G5) 5 is composed of two electrodes 51 and 52.
A focus voltage is applied to the third grid electrode 3 and the first electrode 5, and an anode voltage is applied to the sixth electrode 6, so that an electron beam produced by a triode portion composed of the cathode K, the first grid electrode 1 and the second grid electrode 2 is accelerated and focused by an electron lens formed by the third grid electrode 3 to the sixth grid electrode 6, to project toward a phosphor screen.
Effects on an electron beam of electric fields determined by lengths of, and diameters of electron beam apertures in electrodes of this electron gun differ from electrode to electrode. For example, the shape of the electron beam aperture in the first grid electrode near the cathode K exerts an effect on the spot shape of an electron beam in a small-current region; however, the shape of the electron beam aperture in the second grid electrode exerts an effect on the spot shape of an electron beam in a wide current region from the small-current region to the large-current region.
In the electron gun in which a main lens is formed between the fifth grid electrode 5 and the sixth grid electrode 6 by applying an anode voltage to the sixth grid electrode 6, the shape of the electron beam aperture in each of the fifth grid electrode 5 and the sixth grid electrode 6 forming the main lens exerts a large effect on the shape of electron beam in a large-current region but exerts a smaller effect on the shape of the electron beam in a small-current region than in the large-current region.
The axial length of the fourth grid electrode 4 of the electron gun exerts an effect on the magnitude of the optimum focus voltage and also exerts a large effect on a difference in the optimum focus voltage between a small-current region and a large-current region. The effect of the axial length of the fifth grid electrode 5, however, is significantly smaller than that of the fourth grid electrode 4.
Accordingly, it is required in optimization of each characteristic of an electron beam to optimize the structure of an electrode most effective to the characteristic of the electron beam.
If a shadow mask aperture pitch in the direction perpendicular to the electron beam scanning direction is made smaller or the density of electron beam scanning lines is increased for enhancing resolution in the direction perpendicular to the electron beam scanning direction of a cathode ray tube, an optical interference occurs between electron beam scanning lines and the shadow mask apertures particularly in the electron beam small-current region, and accordingly moire contrast must be suppressed. The prior art, however, fails to solve the above-described problems.
For example, FIGS. 25A and 25B are schematic sectional views each having an essential portion of an electron gun, for comparing the two structures of the electron guns depending on the manner of supplying the focus voltage. FIG. 25A shows a fixed-focus-voltage type electron gun, and FIG. 25B shows a dynamic-focus-voltage type electron gun.
The configuration of the electron gun of the fixed-focus-voltage type shown in FIG. 25A is the same as that shown in FIG. 24, and therefore, parts corresponding to those in FIG. 24 are indicated by the same characters as in FIG. 24.
In the electron gun of the fixed-focus-voltage type shown in FIG. 25A, a focus voltage Vf1 having the same potential is applied to the electrodes 51 and 52 forming the fifth grid electrode 5.
On the other hand, in the electron gun of the dynamic-focus-voltage type shown in FIG. 25B, different focus voltages are respectively supplied to the electrodes 51 and 52 forming the fifth grid electrode 5. In particular, a dynamic focus voltage dVf is supplied to the electrode 52.
In the electron gun of the dynamic-focus-voltage type shown in FIG. 25B, the electrode 52 has a portion extending in the electrode 51. This complicates the structure as compared with the electron gun shown in FIG. 25A, increases the cost of the parts and reduces the efficiency in the assembling process.
FIGS. 26A and 26B are illustrations of waveforms of focus voltages respectively supplied to the electron guns shown in FIGS. 25A and 25B. FIG. 26A shows a focus voltage supplied to the electron gun of the fixed-focus-voltage type and FIG. 26B shows the focus voltage supplied to the electron gun of the dynamic-focus-voltage type.
Specifically, FIG. 26A shows that the fixed focus voltage Vf1 is applied to the third grid electrode 3 and the fifth grid electrode 5 (51, 52), and FIG. 26B shows that the fixed focus voltage Vf1 is applied to the third electrode 3 and the electrode 51 of the fifth grid electrode 5 and a voltage having a waveform of another fixed focus voltage Vf2 superposed with the dynamic focus voltage dVf is applied to the electrode 52 of the fifth grid electrode 5.
As a result, the electron gun of the dynamic-focus-voltage type shown in FIG. 25B requires two stem pins for supplying the focus voltages, and thereby it requires more consideration to high-voltage insulation of the two focus stem pins from the other stem pins as compared with the electron gun of the fixed-focus-voltage type shown in FIG. 25A.
Accordingly, the dynamic-focus-voltage type electron gun requires a specially structured socket for a cathode ray tube in a TV receiver set and a terminal display system, and further it requires a dynamic-focus-voltage generating circuit in addition to the two fixed-focus-voltage power supplies. This causes a disadvantage that it takes a lot of time for adjusting two focus voltages in the assembly line of TV receivers and display terminals.
In a cathode ray tube, the maximum deflection angle (hereinafter referred to as "deflection angle" or "deflection amount") is substantially in a specified range, and accordingly, as the size of a phosphor screen is increased, a distance between the phosphor screen and a main focus lens of an electron gun is increased, as a result of which mutual space-charge repulsion of electrons in such a space aggravates deterioration of focus characteristics.
Accordingly, resolution of a cathode ray tube can be improved by reducing degradation of the focus characteristic caused by space-charge repulsion thereby obtaining a small electron beam spot as in a small size phosphor screen.
The overall length of a cathode ray tube can be shortened by increasing a deflection angle. The depth of the existing TV receiver set (hereinafter referred to as "TV set") is dependent on the overall length of the cathode ray tube, and it is desirable to be short as much as possible because the TV set is regarded as furniture. Shortening of the depth of a TV set is also advantageous in transportation efficiency at the time when a TV set maker transports a large number of TV sets.